The blackbuck: aberrant from gazelles in being so sexually dimorphic that even the anti-predator displays differ?

The adaptive colouration of the blackbuck (Antilope cervicapra) of India is surprising. The female and juvenile male lack the conspicuous dark/pale contrast on the hindquarters that occurs in all species of gazelles (Gazella and Eudorcas), despite the fact that the blackbuck is phylogenetically and ecologically related to the gazelles.

Among gazelles, conspicuous colouration is correlated with gregariousness in open environments, where hiding tends to be impractical and a main anti-predator strategy is instead the flaunting of fitness by means of extravagant displays of locomotory prowess. The hindquarters of gazelles have bold patterns of dark (on tail-tassel and pygal bands) and pale (on posterior haunches and escutcheon, which are usually white), which tend to stand out even when the animals stand still. The display is then heightened by movement of the tail and flaring of the posterior haunch, particularly when the animals stot as a demonstration, to an onlooking predator, of a current capacity to flee so rapidly and enduringly that pursuit of the individual in question would likely be futile.

Everything about the ecology and behaviour of the blackbuck would lead us to expect that the colouration on its hindquarters would be similar to that in gazelles. It is gregarious and lives in treeless grassland. It not only stots but does so more spectacularly and with greater versatility than any gazelle. The blackbuck leaps high, or bounces stiff-legged, or style-trots, and it is capable of flaring the white of the posterior haunches while erecting the tail in the form of an exclamation mark. The mature males are more conspicuous than any gazelle because their whole body turns blackish and white and their sexual displays (which qualify as lekking) are particularly eye-catching.

And yet, contrary to expectations, the non-masculine colouration differs from that of gazelles in significant ways which have yet to be explained. There is barely any pygal band, limiting the contrast between the white of the hindquarters and the fawn of the haunches. The tail lacks a dark tassel. And the tail is usually left inert in gaits and situations in which it would be wagged, swished or erected in gazelles.

What this means is that, in a sense, the blackbucks acts like two different species within one, in adaptive colouration. In the male, the conspicuous pattern on the hindquarters of gazelles has in effect spread to encompass the whole masculine figure, which appears grossly black and white. Meanwhile, in the female and juvenile male the dark emphasis has virtually been scrubbed from the figure. And we are left with the puzzle of how females might possibly benefit from blending into a scene in which their presence is likely to be divulged anyway by the outlandish appearance of the males among them, in a species which breeds throughout the year.

Posted by milewski milewski, March 06, 2021 02:58

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